1. Field of the Invention
The invention relates to a method for picture-in-picture insertion and to a circuit arrangement for picture-in-picture insertion.
2. Description of Related Art
In the case of such picture-in-picture (PIP) insertion, a smaller insertion picture (small picture) is inserted into a larger main picture. The insertion picture is decimated in accordance with the size reduction and is continuously read into a memory device, older stored pictures being overwritten, and then the insertion pictures are read out in a manner employing synchronization with the main pictures. Accordingly, the read-out speed of the insertion pictures is generally higher than the writing speed. In this case, the main picture and the insertion picture may, in a known manner, be fields which are used for displaying a monitor picture.
At specific phase angles of the rasters of the main picture and of the small picture, the higher read-out speed can lead, inter alia, to the read-out pointer overtaking the write pointer and reading out a previous picture stored in the memory device, with the result that a seam occurs in the middle of a displayed small picture and in part the preceding insertion picture is read out. If both insertion pictures originate from different motion phases, a disturbing effect results since moving objects through which the seam runs are displayed with distortion. If the frequencies of the pictures of the insertion channel and main channel correspond only approximately, the result is slow drifting of the disturbance location, which is found to be particularly unpleasant.
EP 0 739 130 A2 describes a method for eliminating this seam by storing two fields of a small picture, with the result that the field that can be read is always exactly the one which is currently not being written, and, consequently, the read pointer cannot overtake the write pointer. A first and a second memory, which each store a field, are provided for this purpose. This method has the disadvantage, however, that a storage capacity of two insertion pictures or fields is necessary, which entails corresponding costs.